With the progress of computer image processing techniques, a method of directing recording images on a photosensitive layer by irradiation corresponding to digital signals has recently been developed and thus an intense interest has been shown toward a computer-to-plate (CTP) system in which images are directly formed on a photosensitive lithographic printing plate, without outputting onto a silver salt mask film, by employing the method in a lithographic printing plate precursor. The CTP system, which uses a high-output laser having a maximum intensity within a near infrared or infrared range as a light source for the irradiation, has the following advantages: images having high resolution can be obtained by exposure within a short time and the photosensitive lithographic printing plate precursor used in the system can be handled in daylight. Regarding solid and semiconductor lasers capable of emitting infrared rays having a wavelength of 760 to 1,200 nm, a high-output and portable laser is available with ease.
Also, as a lithographic printing plate precursor which can form images using a solid laser or semiconductor laser, JP-A-2007-17913 proposes a lithographic-printing plate precursor comprising a substrate, a first image recording layer and a second image recording layer formed on the substrate, wherein the second image recording layer comprises a specific polyurethane so as to prevent scratching of the surface of the lithographic printing plate precursor.
On the other hand, JP-A-2011-170035 proposes a lithographic printing plate precursor comprising a bottom layer comprising a water-insoluble and alkaline-soluble resin and an IR absorber, and a top layer comprising a water-insoluble and alkaline-soluble polyurethane resin and a polyorganosiloxane.